Cell surface molecules play a key role in the infectivity of viruses and other pathogens with their target cells. For example, ICAM-1, the endothelial cell receptor for integrin binding, is also the receptor for rhinovirus binding. Rhinoviruses are members of the picornavirus family and are responsible for about 50% of common colds in humans. Another classic example of such interactions is influenza hemagglutinin, a lectin that binds to sialic acid on its host cell as the first step in infection. Thus, a prophylactic approach to preventing the common cold is to interfere with the binding of rhinovirus to cell-bound ICAM-1 by administration of soluble binding competitors to the host receptor.
Lectins are a class of proteins that bind carbohydrates specifically and noncovalently. Lis, H. and Sharon, N., 1986, Annual Review of Biochemistry, 55:35. Numerous lectins have been identified in higher animals, both membrane-bound and soluble, and have been implicated in a variety of cell-recognition phenomena, in addition to roles that they play in metastasis.
Lectins may generally be classified as either of the C-type, whose binding properties are calcium-dependent and which are structurally related to the asialoglycoprotein receptor, or the S-type, or thiol-dependent lectin. It should be noted, however, that there are other proteins with lectin properties that apparently do not fall into either of these classes, such as for example, fibronectin and laminin. Drickamar, K., 1988, J. Biol. Chem., 263:9557.
Lectins are thought to play a role in regulating cellular events that are initiated at the level of the plasma membrane. For instance, plasma membrane associated molecules are involved in the activation of various subsets of lymphoid cells, particularly T-lymphocytes, and it is known that cell-surface molecules are responsible for activation of these cells and consequently their response during an immune reaction. This phenomenon has been studied using various plant lectins, such as leucoaggutinating phytohaemagglutinin (PHA) and concanavalin A (con A). These molecules are thought to activate T-cells by binding to carbohydrate moieties associated with specific molecules on the T-lymphocyte cell surface.
One known human lectin, originally described as a cell-associated macrophage antigen, is called "Mac-2". Ho & Springer, J. Immunol., (1982) 128:1221-1228. The 32-kDa lectin Mac-2, is expressed at significant levels in thioglycolate-elicited mouse macrophages but not in resident macrophages and may be involved in cell adhesion or immune responses. The human homologue of Mac-2 has been cloned and shown to be a lactose/galactose-specific lectin that is externalized, despite lacking a leader sequence. Oda et al., Gene (1991) 99:279-283; Cherayil et al., PNAS (USA) (1990) 87:7324-7438. Mac-2 is identical or closely related to other previously described lectins: EBP, a protein believed to represent a new type of cell adhesin (Frigeri & Liu, J. Immunol. (1992) 148:861-867), CBP35, a galactose-binding lectin (Jai & Wang, J. Biol. Chem. (1988) 263:6009-6011), a non-integrin laminin-binding protein (Woo et al., J. Biol. Chem. (1990) 265:7097-7099), RL-29, a lactose-specific lung lectin (Leffier & Barondes, J. Biol. Chem. (1986) 261:10119-10126), and L-34, a galaclose-binding lectin correlated with neoplastic transformation and metastasis (Raz et al., Int. J. Cancer (1990) 46:871-877). Mac-2 is present in significant concentrations in the tips of intestinal villi, where it may be a target for colonization by human pathogens.
Several researchers have isolated and purified glycoproteins that act as ligands for various lectins. For example, a protein termed the tamm-horsfall glycoprotein has been shown to inhibit lymphocyte activation induced by several lectins, including leucoagglutinin and hemagglutinin from Phaseolus vulgaris. Serafini-Cessi, F. et al., 1979, Biochemical Journal, 183:381-388. Additionally, glycoproteins that act as PHA-binding factors have been partially purified from porcine splenic lymphocytes. Further studies also show that PHA activation of porcine lymphocytes is inhibited by the partially purified glycoproteins. Dupuis, G. et al., 1985, Canadian Journal of Biochemistry & Cell Biology, 63:932-940. These researchers were unable to identify a precise molecular weight species that exhibited the inhibitory activity. Rather, they reported a range of molecular weight species in partially purified preparations, as revealed by Coomassie Blue staining of sodium dodecyl sulphate polyacrylamide gels. Major bands were observed having apparent molecular weights of about 50-55, 75, 95, 130, and 155 kD; additional minor species exhibited apparent molecular weights of about 42, 45, 60-65, 175, and 200-250 kD.
Similar studies by other investigators have shown the existence of other PHA-binding molecules. For instance, a PHA-binding factor from pig mesenteric lymph nodes has been isolated and shown to have a molecular mass of about 100 kD. Allan, D. and Crumpton, N. J., 1973, Exp. Cell Res., 78:271-278. A PHA-binding molecule present in plasma membranes from pig submaxillary lymph node lymphocytes was shown to exhibit an apparent molecular mass greater than 94 kD. Alexander, S. et al., 1978, Biochemical Biophys. Acta, 512:350-364. Other PHA-binding ligands have been isolated from human peripheral blood by affinity chromatography and have been found to have molecular masses in the range of 20-35, 43, 60, and 70 kD. Skoog, B. et al., 1980, Scand. Journal Immun., 11:369-376. Other researchers have reported the presence of leucoagglutinin-receptor glycoproteins with molecular weights ranging from 43 to 250 kD in neuraminidase-treated peripheral human T lymphocytes.
The health benefits of human breast milk have long been recognized. Recently, the prophylactic effects of milk in preventing gastrointestinal infections have been described. Gerrard, J., 1974, Pediatrics, 54:757-764. At least in part, this is due to non-immunoglobin glycoproteins present in milk that have binding properties that protect newborns from viral or bacterial infections. Lonnerdal, B., 1985, Am. J. Clin., 42:1299-1317 and Holmgren, J. et al., 1983, Infect. Immun., 33:459-463. These proteins are thought to exert their effects, at least in part, by preventing or disrupting the adherence of bacteria or viruses to intestinal epithelium by binding to bacterial adhesins or viral hemagglutinins. Bacterial adhesins and viral hemagglutinins are surface molecules that facilitate the adherence of these organisms to epithelial cell surfaces as an early step in infection. The milk proteins involved in this process have not been well characterized. Holmgren, J. et al., 1983, Infect. Immun., 33:459-463. However, a glycoprotein having a molecular weight above 400,000 has been described that is able to neutralize respiratory syncytial virus. Laegreid, A. et al., 1986, Acta Paediatric. Scand., 75:696-701. In addition to the immuno-protective and anti-infective functions provided by such proteins, other human milk proteins serve special roles as carriers of specific nutrients.
Similarly, a high molecular weight material has been identified in human serum that interferes with the attachment and infectivity of hepatitis A virus to various cell lines. Zajac, A. et al. 1991, J. of Gen. Virol, 72:1667-1675. This material has not been purified nor have its properties been further characterized.
Rosenberg et al., J. Biol. Chem. (1991) 266:18731-18736, describe a Mac-2 binding protein from colon carcinoma cells, and report a partial N-terminal protein sequence. Linsley et al., Biochem. (1986) 25:2978-2986 have characterized a lung carcinoma protein, L3, with a similar, but not identical N-terminal protein sequence. No one has as yet identified the complete amino acid sequence for a novel glycoprotein specific for binding to the human Mac-2 lectin, nor has the cDNA encoding such a sequence been cloned.